The present invention relates to a method for manufacturing surface-treated printing paper, in which method the surface of the paper is treated after drying.
In accordance with such a method, paper with certain desired properties is produced in an advantageous manner.
The invention also relates to an apparatus for manufacturing surface-treated printing paper in which apparatus the paper is treated after drying.
A papermaking method of prior art is the one in which finished calendered paper web is produced in separate processes. Thus, the web is first dried in the paper machine, typically down to the moisture of approximately 2 to 4%, i.e. it is overdried, whereafter the paper is wetted typically to the moisture of 8 to 12% before reeling up the web. Thereafter, when the paper is on the reel, the moisture has the time to equalize itself in the thickness direction of the paper before surface treatment in the calender, i.e. calendering. In this context, overdrying refers to the process of drying the paper into a moisture which is lower than the moisture allowed normally when the paper enters the calender nip, when it is taken into account that in the calendering the moisture of the web is reduced further by over 2 percentage points, wherein the paper attained after calendering is drier than the moisture of use of the paper in normal conditions.
Another papermaking method of prior art is the one in which in connection with the drying of paper, e.g. in the drying section, the web is wetted with the aim of correcting the moisture profile of the paper.
It has been suggested that the calendered paper could be manufactured in such a way that calendering is conducted in connection with the manufacturing process, wherein the web is dried directly down to a so-called calendering moisture, i.e. to a target moisture of calendering.
However, prior art entails considerable problems and drawbacks. Especially in the manufacturing processes of SC paper grades it is, in addition to the wetting of paper, important how moisture is controlled before wetting to the target moisture of calendering.
Thus, the purpose of the present invention is to eliminate the drawbacks of prior art and to attain an entirely new solution in which the papermaking process is effected in an optimal way.
The invention is primarily based on the idea that at the same time when a paper web is produced by means of a papermaking process, the paper web is first dried in a drying section or the like down to a determined first moisture content, whereafter the moisture content of the web is increased to a second treatment moisture content, whereafter the web is guided substantially immediately to the treatment of paper in which its surface properties are affected, after which the web is reeled on paper reels formed thereby.
Within the scope of the present invention, the moisture of the web refers to the ratio of its water content to the entire mass.
According to a first advantageous embodiment of the invention, the manufacture of paper is conducted as a continuous process in the following way. The formation and mechanical dewatering of paper is conducted in a preselected way. The web which is thereby formed is dried further so that the moisture content of the web is reduced in a desired way to a particular first moisture content, whereafter the moisture of the web is increased to a second, treatment moisture content of the web which, in other words, is higher than the first moisture content, whereafter the web is guided substantially immediately to paper treatment in which its surface properties are affected. Thereafter the paper is reeled in the reel-up to form a reel.
In this context, the substantially immediate guiding of the web refers to a wetting location by means of which a desired residence time from the wetting to the treatment of the paper is attained. Advantageously this is determined in such a way that the residence time from the wetting to the treatment will be 0.2 to 2.0 seconds. The lower limit is primarily determined by the fact that the aim is to ensure an even moisture distribution in the paper and the upper limit by the penetration of moisture in the paper. The machine construction is also a determining factor for the upper limit because the path travelled by the web from the wetting to the treatment becomes long.
Within the scope of this application, the act of affecting the surface properties advantageously involves calendering or production of a corresponding effect. After the paper has been dried, the surface structure of the web is made suitable by means of a mechanical treatment, calendering. There are several calendering methods, but it is common to all of them that the web is passed through one or several nips which are formed between two surfaces, typically between rotating roll surfaces. The purpose of the calendering is to improve the paper quality by pressing the paper into a fixed final thickness, and especially by smoothing its surface. As is well known, moisture improves the mouldability of fibres contained in paper in connection with calendering. Because of this, the calendering is conducted in a certain moisture content, xe2x80x9ctarget moisture of the calenderingxe2x80x9d.
Advantageously the increase of moisture from the first moisture content to the second moisture content before affecting the surface properties of paper, e.g. before calendering, is not higher than 10%, advantageously 2 to 10%, expressed as a difference between the moistures.
According to a second advantageous embodiment of the invention, the manufacture of paper is conducted in a continuous process in such a way that the formed web is dried in such a way that the moisture content of the web is reduced in a desired manner to a certain first moisture content, advantageously to a moisture of xe2x89xa66%, typically, depending on the case, to a moisture of 2 to 6%, whereafter the moisture of the web is increased to a second, treatment moisture content of the web, which is preferably xe2x89xa78%, most preferably 8 to 12%, i.e. it is higher than the first moisture content, whereafter the web is guided substantially immediately to paper treatment in which its surface properties are affected.
By means of the invention considerable advantages are achieved. First of all, the dimensional stability of the paper is improved when the fibres experience the first drying stage. This xe2x80x9coverdryingxe2x80x9d is well founded because the moisture expansion potential of the paper is reduced when the lowest moisture content experienced by the paper during the papermaking is reduced. This means that the behaviour of the web will be more stable for example in printing. On the other hand, drying to the first moisture content levels down possible moisture streaks in the paper. In the method according to the invention, it is especially important to level down small-scale (1 to 3 mm) streaks.
Another essential aspect in the process according to the invention is that the moisture profile of the paper web to be formed is sufficiently straight. Thus, a feature entailed in the solution according to the invention is a method for controlling the moisture profile in the papermaking process. Conventionally, a known procedure has been the use of a so-called profiling steam box in the press section and at a later stage a profiling water moistener as an actuator in the adjustment of the moisture profile. It has now been observed that with respect to the entire process, there is a certain stage in which it is not advantageous to utilize wetting. It has been detected that there is a stage in the drying of the web where the web shrinks strongly primarily in the cross machine direction, and it has been observed that the use of wetting in this area may have disadvantageous effects. Typically, the web shrinks strongly in the cross machine direction when in the dry matter content range of 65 to 85%. This area can vary e.g. according to the conditions of the process and the paper grade to be manufactured.